Dropping the krb protocol check for CTA admins

Before this commit, Daniele had hardcoded into the CTA
front-end the constraint that CTA admins must be
authenticated by krb5.

This commit drops this contraint for two reasons:

1. The xrootd/EOS configuration files already require
   operators to specify which authentication methods can
   be used.  Daniele’s constraint is not needed to
   further enforce security, it only serves to constrain
   admin authentication to krb5 which I personally do not
   see as providing any added value.

2. The CERN infrastructure uses krb5, other sites like
   RAL do not.